The present invention relates to a double heterojunction semiconductor laser and, more particularly, to a semiconductor laser in which light confinement is improved to reduce divergence of the laser light beam produced by the laser.
FIG. 9A is a partial perspective view of a double heterojunction semiconductor laser according to the prior art. The semiconductor laser includes an n-type GaAs substrate 1 on which are successively disposed an n-type Al.sub.0.5 Ga.sub.0.5 As cladding layer 2, an active layer 3, and a p-type Al.sub.0.5 Ga.sub.0.5 As cladding layer 4. The active layer 3 has a multiple quantum well (MQW) structure, shown in detail in FIG. 9B. The MQW layer 3 includes, alternatingly laminated, InGaAs well layers 6 and Al.sub.0.2 Ga.sub.0.8 As barrier layers 7. The outside layers of the active layer 3 are InGaAs barrier layers. These laminated layers 6 and 7 are, in turn, sandwiched between a pair of AlGaAs guide layers 5.
In the course of manufacturing the semiconductor laser of FIG. 9A, after the growth of the cladding layers and the active layer, preferably by metal organic chemical vapor deposition (MOCVD), the cladding layer 4 is masked and etched to form a ridge 8 extending in the resonator length direction of the semiconductor laser and having a top surface with a width of about 2 microns and oblique side surfaces. Subsequently, in another MOCVD step, an n-type Al.sub.0.7 Ga.sub.0.3 As layer 9 is grown on the cladding layer 4 next to and on both sides of the ridge 8, contacting side surfaces of the ridge, as a current blocking layer. A p-type GaAs contact layer 10 is subsequently grown on the top surface of the ridge 8 and on the current blocking layer 9. The laser is completed by forming electrodes 11 and 12 on the contact layer 10 and the substrate 1, respectively. Because of the pn junction formed between the cladding layer 4 and the current blocking layer 9, current flow between the electrodes 11 and 12 is concentrated in the ridge 8. The current concentration ensures laser oscillation of the semiconductor laser at a relatively low voltage by producing a current density that exceeds the threshold current density required for laser oscillation.
The lower half of FIG. 10 is a graph of the refractive index of the semiconductor laser structure of FIG. 9A taken along an imaginary line passing through the center of the laser of FIG. 9A perpendicular to the electrodes 11 and 12. The relative distance along that line is plotted on the abscissa of FIG. 10. As shown in that graph, the refractive index of the cladding layers 2 and 4 is smaller than that of the active layer 3 so that light is generally confined to the active layer 3 by the cladding layers. The cladding layers 2 and 4 separate the active layer 3 from GaAs, i.e., the substrate 1 and the contact layer 10. Although the cladding layers 2 and 4 significantly confine light to the active layer 3, there is still some light leakage to the substrate and the contact layer.
The light generated within the laser propagates through the layers of the laser producing a light distribution along the same direction of the graph of refractive index, as indicated in the upper half of FIG. 10. There, relative light intensity as a function of position is plotted. Although most light is confined to the active layer 3, some light reaches the substrate 1 and the contact layer 10. The relatively small amplitude variations in light intensity shown in the substrate 1 and the contact layer 10 result from light leakage into and resonance in those GaAs elements. The light leakage increases the divergence of the laser beam produced by the laser in the far field pattern in a plane transverse to the plane of the active layer 3 of the semiconductor laser. Therefore, it is desirable to prevent light leakage and/or resonance in the semiconductor laser perpendicular to the layers in order to avoid undue divergence of the light beam produced by the laser.
In order to reduce light leakage into the substrate and contact layers, the thickness of the ridge 8, of the other portions of the cladding layer 4, and of the cladding layer 2 might be increased. Although the increased thicknesses of the layers increase light attenuation, reducing light leakage, several problems result from increasing the thicknesses of the cladding layers. For example, if the height of the Al.sub.0.5 Ga.sub.0.5 As ridge 8 is too large, the crystalline lattice mismatches between the cladding layer 4 and the Al.sub.0.7 Gao.sub.0.3 As current blocking layer 9 and between the current blocking layer and the contact layer 10 adversely affect the crystallinity of those layers. The lattice mismatch can be accommodated if the dimensions of the mismatched layers are limited but not if those dimensions are too long. The same limitation applies to the thickness of the cladding layer 2 which has a different lattice constant from the GaAs substrate 1. Poor crystallinity resulting from lattice constant mismatches that are not accommodated by the semiconductor materials adversely affect the electrical and optical characteristics of the semiconductor laser.